1. Field of the Invention
The present invention relates to a laser engravable flexographic printing form precursor for laser engraving. The present invention also relates to a method of forming a flexographic printing form by laser engraving.
2. Description of the Related Art
Flexography is today one of the most important processes for printing and commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Packaging foils and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography, which indeed makes it very appropriate for packaging material printing. It uses a rubber printing plate or a flexible photopolymer plate that carries the printing image in relief. The ink delivery system for flexography is achieved via an “anilox” engraved ink transfer roll.
Analogue flexographic printing forms are prepared from printing form precursors including a photosensitive layer on a support or substrate. The photosensitive layer includes ethylenically unsaturated monomers or oligomers, a photo-initiator and an elastomeric binder. The support preferably is a polymeric foil such as PET or a thin metallic plate. Image-wise crosslinking of the photosensitive layer by exposure to ultraviolet and/or visible radiation provides a negative working printing form precursor which after development with a suitable developer (aqueous, solvent or heat development) leaves a printing relief, which can be used for flexographic printing. Imaging of the photosensitive layer of the printing form precursor with ultraviolet and/or visible radiation is typically carried out through a mask, which has clear and opaque regions. Crosslinking takes place in the regions of the photosensitive layer under the clear regions of the mask but does not occur in the regions of the photosensitive layer under the opaque regions of the mask. The mask is usually a photographic negative of the desired printed image. Flexographic printing form making according to the above described process has the disadvantage that the production of a mask is time consuming and that the dimensional stability of these masks with changing environmental temperatures or humidities is unsatisfactory for high quality printing and color registration. Moreover, the use of separate masks in flexographic printing form production means additional consumables and chemistry, with a negative impact on economy and ecology aspects of the production process, which are far more a concern than the additional time required for making the masks. As a matter of fact, in most cases plate exposure and plate development may turn out to be more time consuming than mask making.
Digital imaging using laser recording of printing form precursors, which eliminates the making of a separate film mask, is becoming increasingly important in the printing industry. The flexographic printing form precursor is made laser-sensitive by providing e.g., a thin, for UV and visual radiation opaque, IR-sensitive layer on top of the photopolymerizable layer of the flexographic printing form precursor. Such a flexographic printing form, also referred to as flexographic printing plate precursor, is sometimes called a “digital” or “direct-to plate” flexographic printing plate precursor. An example of such a “direct-to-plate” flexo plate precursor is disclosed in EP-A 1 170 121. The thickness of the IR-ablative layer(s) is usually just a few μm. The IR-ablative layer is inscribed image-wise using an IR laser, i.e., the parts in which the laser beam is incident on it are ablated, i.e., removed. The actual printing relief is produced in the conventional manner: exposure with actinic light (UV, visible) through the mask, the mask being image-wise opaque to the crosslinking inducing light, resulting in an image-wise crosslinking of the photopolymerizable layer, i.e., relief forming layer. Development with an organic solvent, water or heat removes the photosensitive material from the unexposed parts of the relief forming layer and the residues of the IR-ablative layer. Development may be performed using different developing steps or a single developing step. Since this method still requires a developing step, the improvement in efficiency for producing flexo printing forms is limited.
In the direct laser engraving technique for the production of flexographic printing forms, a relief suitable for printing is engraved directly into a layer suitable for this purpose. By the action of laser radiation, layer components or their degradation products are removed in the form of hot gases, vapors, fumes, droplets or small particles and nonprinting indentations are thus produced. Engraving of rubber printing cylinders by means of lasers has been known since the late 60s of the last century. However, this technique has acquired broader commercial interest only in recent years with the advent of improved laser systems. The improvements in the laser systems include better focusing ability of the laser beam, higher power, multiple laser beam or laser source combinations and computer-controlled beam guidance. Direct laser engraving has several advantages over the conventional production of flexographic printing plates. A number of time-consuming process steps, such as the creation of a photographic negative mask or development and drying of the printing plate, can be dispensed with. Furthermore, the sidewall shape of the individual relief elements can be individually designed in the laser engraving technique.
A problem associated with direct laser engraving is the formation of debris. The debris is presumed to be decomposition products of the resin of the precursor formed by the action of the laser. Gaseous or particulate ablated material is, at least partially, removed by an exhaust fan. However, liquefied ablated material may be very difficult to be removed. The generation of such debris not only necessitates a time consuming treatment to remove the debris, but may result in coarse and irregular edged relief shapes. Moreover, when a large amount of liquid debris is generated during laser engraving, the debris may stain the optical parts of the laser engraving apparatus, necessitating a time consuming cleaning procedure or an expensive replacement of the laser optics.
EP-A 1 424 210 discloses a photosensitive resin composition for a laser engravable precursor including (1) 100 parts by weight of a resin which is a plastomer at 20° C. wherein the resin has a number average Molecular Weight (Mw) of from 1,000 to 100,000 and has at least 0.7 of an average number of polymerizable groups per molecule, (2) 5 to 200 parts by weight, relative to 100 parts by weight of the resin (1) of an organic compound having a Mw of less than 1,000 and having at least one polymerizable group per molecule and (3) 1 to 100 parts by weight, relative to 100 parts by weight of the resin (1) of an inorganic porous material. The inorganic porous material, preferably porous silica products, is used to absorb the liquid debris formed upon laser engraving of the cured resin composition.
US 2005/0227165 discloses a photosensitive resin composition for a laser engravable precursor including (1) 100 parts by weight of a resin which is in a solid state at 20° C. wherein the resin has a number average Molecular Weight (Mw) of from 5,000 to 300,000, (2) 5 to 200 parts by weight, relative to 100 parts by weight of the resin (1) of an organic compound having a Mw of less than 5,000 and having at least one polymerizable group per molecule and (3) 1 to 100 parts by weight, relative to 100 parts by weight of the resin (1) of an inorganic porous material having an average pore diameter of from 1 nm to 1,000 nm, a pore volume of from 0.1 ml/g to 10 ml/g and a number average particle diameter of not more than 10 μm. The inorganic porous material, preferably porous silica products, is used to absorb the liquid debris formed upon laser engraving of the cured resin composition.
EP-A 1 710 093 discloses a photosensitive resin composition for a laser engravable precursor including a (1) a resin having polymerizable groups and a number average Molecular Weight (Mw) in the range of 1,000 to 200,000, (2) an organic compound having a polymerizable group and a Mw of less than 1,000 and (3) an organo-silicon compound having at least one Si—O bond. The organo-silicon compound does not have unsaturated polymerizable groups and is present in an amount of 0.1 to 10% by weight relative to the total photosensitive resin composition.
EP-A 539 227 discloses an aqueous developable composition suitable for preparing flexographic printing forms, including (1) a polymerizable material and (b) a hydrophobic compound containing an element selected from the group consisting of fluorine, chlorine and silicon, the hydrophobic compound being capable of copolymerizing with the polymerizable composition. The presence of the hydrophic compound reduces “plugging”, i.e., build up of foreign material such as ink between relief parts of the plate during printing. The flexographic printing form of EP-A 539 227 is prepared by aqueous development after image-wise exposure of the precursor through a negative mask.
JP 2005-254696 A discloses a flexographic printing form including a cylindrical substrate and a curable composition provided thereon wherein the photocurable composition contains an organo-silicon or organo-fluor compound. The printing form has a Shore D hardness of from 30 to 100 making the printing form suitable for intaglio printing. A flexographic printing form having a Shore D hardness of more than 30 (equivalent with a Shore A hardness of more than 80) is outside the scope of the present invention.